CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 98116863, filed on May 21, 2009. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an image processing circuit and an image processing method. More particularly, the present invention relates to an image processing circuit and an image processing method that perform an image sharpening operation.

2. Description of Related Art

When a video is played, color and tone are not the only characteristics that draw observers' attention. Edges, which define the shapes of objects in the video, also attract observers' attention. Image processing is a kind of operation that enhances those video characteristics and improves or emphasizes visual effects of the video. For example, post procedure can make images captured by digital cameras or scanners become clearer and more colorful.

Sharpening operation is an important image processing technique. â€œSharpnessâ€ is a term that people frequently use to indicate a contract ratio. After sharpening the edges of an object in an image, the image's contrast ratio increases, and the object becomes more conspicuous. After performing a sharpening operation on an original image to generate a sharpened image, observers generally deem that the sharpened image is clearer than the original image.

The sharpening operation can enhance the edges of the objects in the images. However, it can also result in some undesired visual effects. For example, in a border area between different colors, such as between white cloud and blue sky, sharpening operation may generate discernible halo and decrease the visual quality of the image.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides an image processing circuit and an image processing method, thereby receiving an input video signal of a plurality of pixels, generating a weighting signal through calculating color difference values among the pixels, and performing a sharpening operation on the input video signal according to the weighting signal.

The present invention provides an image processing circuit that includes an adaptor and a sharpening circuit. The adaptor includes a processing unit and a weight generator. The processing unit receives an input video signal that includes information of a plurality of pixels. According to chromatic information of at least two pixels adjacent to a target pixel, the processing unit processes the input video signal to determine a color difference value of a target pixel of the pixels. The weight generator is coupled to the processing unit and generates a weighting signal according to the color difference value. The weighting signal includes a weighting value of the target pixel. The sharpening circuit is coupled to the adaptor and performs a sharpening operation on the input video signal according to the weighting signal to generate a sharpened video signal.

The present invention provides an image processing method that includes following steps: receiving an input video signal that includes information of a plurality of pixels, processing the input video signal to determine a color difference value of a target pixel of the pixels according to chromatic information of at least two pixels adjacent to the target pixel, generating a weighting signal which includes the weighting value of the target pixel according to the color difference value, and performing a sharpening operation on the input video signal according to the weighting signal to generate a sharpened video signal.

According to an embodiment of the present invention, the chromatic information of each of the at least two adjacent pixels includes a first chrominance Cb and a second chrominance Cr.

According to an embodiment of the present invention, the processing unit calculates the color difference value of the target pixel according to a first chrominance and a second chrominance of a first pixel and a first chrominance and a second chrominance of a second pixel. The first and the second pixels are adjacent to the target pixel.

According to an embodiment of the present invention, the color difference value equals (|Cb(1)âˆ’Cb(2)|+|Cr(1)âˆ’Cr(2)|)/2. Cb(1) and Cr(1) are a first chrominance and a second chrominance of the first pixel, respectively. Cb(2) and Cr(2) are a first chrominance and a second chrominance of the second pixel, respectively.

According to an embodiment of the present invention, the first chrominance Cb and the second chrominance Cr serve as two reference axes of a chromatic coordinate system. The chromatic coordinate system includes four regions {(Cb, Cr)|Cbâ‰¦Cb2, Crâ‰¦Cr2}, {(Cb, Cr)| Cbâ‰§=Cb1, Crâ‰¦Cr2}, {(Cb, Cr)|Cbâ‰¦Cb2, Crâ‰§Cr1}, and {(Cb, Cr)|Cbâ‰§Cb1, Crâ‰§Cr 1}, where Cb2>Cb1>0, and Cr2>Cr1>0. The first and the second chrominances of the first pixel are Cb(1) and Cr(1), respectively. The first and the second chrominances of the second pixel are Cb(2) and Cr(2), respectively. The possessing unit determines the color difference value according to the region(s) where coordinates (Cb(1),Cr(1)) and (Cb(2),Cr(2)) are located.

According to an embodiment of the present invention, when the coordinates (Cb(1),Cr(1)) and (Cb(2),Cr(2)) are located in a same region of the four regions, the color difference value equals 0.

According to an embodiment of the present invention, when the coordinates (Cb(1),Cr(1)) and (Cb(2),Cr(2)) are located in two different regions of the four regions, the color difference value equals âˆš{square root over ((Cb(1)âˆ’Cb(2))²+(Cr(1)âˆ’Cr(2))²)}{square root over ((Cb(1)âˆ’Cb(2))²+(Cr(1)âˆ’Cr(2))²)}{square root over ((Cb(1)âˆ’Cb(2))²+(Cr(1)âˆ’Cr(2))²)}{square root over ((Cb(1)âˆ’Cb(2))²+(Cr(1)âˆ’Cr(2))²)}.

According to an embodiment of the present invention, when the coordinates (Cb(1),Cr(1)) and (Cb(2),Cr(2)) are located in two different regions of the four regions, the color difference value equals (|Cb(1)âˆ’Cb(2)|+|Cr(1)âˆ’Cr(2)|).

According to an embodiment of the present invention, the target pixel is located at the middle point of the first and the second pixels.

According to an embodiment of the present invention, the weighting value is a monotonic decreasing function of the color difference value.

According to an embodiment of the present invention, the weight generator interpolates the weighting value of the target pixel according to at least two predetermined color difference values and corresponding weighting values.

According to an embodiment of the present invention, the weight generator looks up the weighting value of the target pixel from a lookup table according to the color difference value.

According to an embodiment of the present invention, the sharpening circuit includes a first multiplier. The first multiplier is coupled to adaptor and multiplies the weighting value of the target pixel by a gain. The sharpening circuit performs the sharpening operation on the input video signal according to the weighting signal and the gain to generate the sharpened video signal.

According to an embodiment of the present invention, the sharpening circuit includes a high-pass filter, a coring circuit, a second multiplier, a delay unit, and an adder. The high-pass filter receives the input video signal and high-pass filters the input video signal to generate a high frequency video signal. The coring circuit is coupled to the high-pass filter and eliminates a core portion of the high frequency video signal to generate a coring video signal. The second multiplier is coupled to the weight generator and the coring circuit and multiplies the coring video signal by the weighting signal to generate a sharpened signal. The delay unit receives and delays the input video signal to generate a delayed video signal. The adder is coupled to the second multiplier and the delay unit and adds up the sharpened signal and the delayed video signal to generate the sharpened video signal.

According to an embodiment of the present invention, the sharpening circuit includes a low-pass filter, a coring circuit, a second multiplier, a delay unit, and a subtractor. The low-pass filter receives and low-pass filters the input video signal to generate a low frequency video signal. The coring circuit is coupled to the low-pass filter and eliminates a core portion of the low frequency video signal to generate a coring video signal. The second multiplier is coupled to the weight generator and the coring circuit and multiplies the coring video signal by the weighting signal to generate a sharpened signal. The delay unit receives and delays the input video signal to generate a delayed video signal. The subtractor is coupled to the second multiplier and the delay unit and subtracts the sharpened signal from the delayed video signal to generate the sharpened video signal.

According to an embodiment of the present invention, the processing unit receives the input video signal and determines the color difference values of the pixels in the input video signal. The weight generator generates the weighting signal according to the color difference values of the pixels. The weighting signal includes the weighting values of the pixels. The weighting value of each of the pixels is determined according to the color difference value of the pixel. Generally, pixels on the border area of different colors have relatively larger color difference values. Therefore, the weight generator adaptively adjusts the weighting value according to the color difference value of each of the pixels. Since the sharpening circuit adjusts the degree of the performed sharpening operation according to the weighting signal, the sharpened video signal has a better visual quality.

In order to the make the aforementioned and other features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 is a block diagram of an image processing circuit according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing some pixels of an input video signal according to an embodiment of the present invention.

FIG. 3 is a diagram showing a chromatic coordinate system according to an embodiment of the present invention.

FIG. 4 is a coordinate diagram showing the relationship between a color difference value and a weighting value according to an embodiment of the present invention.

FIG. 5 is a block diagram of the sharpening circuit of FIG. 1.

FIG. 6 is a partial timing diagram of a high frequency video signal according to an embodiment of the present invention.

FIG. 7 is a partial timing diagram of a coring video signal according to an embodiment of the present invention.

FIG. 8 is a block diagram of the sharpening circuit of FIG. 1.

FIG. 9 is a block diagram of the sharpening circuit of FIG. 1.

FIG. 10 is a block diagram of the sharpening circuit of FIG. 1.

FIG. 11 is a flow chart of an image processing method according to an embodiment of the present invention.

FIGS. 12Ëœ14 are flow charts of generating a sharpened video signal according to an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Please refer to FIG. 1, which is a block diagram of an image processing circuit according to an embodiment of the present invention. The image processing circuit 100 receives an input video signal S_INand performs a sharpening operation on the input video signal S_INto generate a sharpened video signal S_our. The image processing circuit 100 includes an adaptor 101 and a sharpening circuit 103. The adaptor 101 includes a processing unit 105 and a weight generator 107. The processing unit 105 receives the input video signal S_INwhich includes information of a plurality of pixels. Please refer to FIG. 2, which is a schematic diagram showing some pixels of the input video signal according to an embodiment of the present invention. Pixels P_Nâˆ’3ËœP_N+3are a series of pixels in the input video signal S_IN, where N is a positive integer, and P_Nis the target pixel.

Please refer to both FIG. 1 and FIG. 2. The processing unit 105 processes the input video signal S_INto determine color difference values Î”C of the pixels. In the input video signal S_IN, the chromatic information of each pixel includes a first chrominance Cb and a second chrominance Cr. In other words, Cb(Nâˆ’3)ËœCb(N+3) represent the first chrominances of the pixels P_Nâˆ’3ËœP_N+3, respectively; Cr(Nâˆ’3)ËœCr(N+3) represent the second chrominances of the pixels P_Nâˆ’3ËœP_N+3, respectively.

The color difference value Î”C of each pixel can be determined according to the chromatic information of at least two pixels adjacent to the pixel. For simplicity of explanation, in this embodiment the color difference value Î”C of each pixel is determined according to the chromatic information of two adjacent pixels. For example, to determine a color difference value Î”C_Nof the target pixel P_N, the processing unit 105 can choose a first pixel P_Nâˆ’3and a second pixel P_N+3, which have the same distances from the target pixel P_N. In another embodiment, the processing unit 105 can choose pixels P_N-2and P_N+₂as the first pixel and the second pixel, respectively. In yet another embodiment, the processing unit 105 can choose pixels P_Nâˆ’1and P_N+1as the first pixel and the second pixel, respectively. The processing unit 105 then calculates the color difference value Î”C_Nof the target pixel P_Naccording to the chromatic information of the chosen first and second pixels. Please note that the processing unit 105 can also calculate the color difference value Î”C_Nof the target pixel P_Naccording to the chromatic information of more than two, e.g. four, pixels adjacent to the target pixel P_N.

As mentioned above, the processing unit 105 can calculate the color difference value Î”C_Nof the target pixel P_Naccording to the first and the second chrominances of the first pixel P_Nâˆ’3and the first and the second chrominances of the second pixel P_N+3. The following embodiment provides a way to calculate the color difference value Î”C_N. First, a first color difference value |Cb(Nâˆ’3)âˆ’Cb(N+3)| and a second color difference value |Cr(Nâˆ’3)âˆ’Cr(N+3)| are calculated. Next, the average value of the two color difference values is calculated to obtain the color difference value Î”C_N=(|Cb(Nâˆ’3)âˆ’Cb(N+3)|+|Cr(Nâˆ’3)âˆ’Cr(N+3)|)/2.

The following embodiment provides another way to calculate the color difference value Î”C_N. Please refer to FIG. 3, which shows a chromatic coordinate system according to an embodiment of the present invention. The first chrominance Cb and the second chrominance Cr serve as the reference axes of the chromatic coordinate system 300. The chromatic coordinate system 300 has four regions including region R1, region R2, region R3, and region R4. In this embodiment, the regions R1ËœR4 are defined as: {(Cb, Cr)|Cbâ‰¦Cb2, Crâ‰¦Cr2}, {(Cb, Cr)|Cbâ‰§Cb1, Crâ‰¦SCr2}, {(Cb, Cr)|Cbâ‰¦Cb2, Crâ‰§Cr1}, and {(Cb, Cr)|Cbâ‰§Cb1, Crâ‰§Cr1}, respectively. Thresholds Cb1, Cb2, Cr1, and Cr2 are used to define the regions R1ËœR4, where Cb2>Cb1>0, and Cr2>Cr1>0. Please note that in this embodiment, each two regions of the four regions have an overlap area. For example, regions 301 and 302 constitute the overlap area of the regions R1 and R3.

In this embodiment, the color of the region R1 is close to green, the color of the region R2 is close to blue, the color of the region R3 is close to red, and the color of the region R4 is close to pink. Please note that the overlap area of every two regions has a color close to that of the two regions. For example, since the regions 301 and 302 constitute the overlap area of the regions R1 and R3, the color of the overlap area is close to both green and red. In addition, the difference between the thresholds Cb1 and Cb2 and the difference between the thresholds Cr1 and Cr2 correlate to the brightness of the chromatic coordinate system 300. The higher the brightness, the smaller the differences; the lower the brightness, the larger the differences.

In the chromatic coordinate system 300, the coordinate of the first pixel P_Nâˆ’3is (Cb(Nâˆ’3),Cr(Nâˆ’3)), and the coordinate of the second pixel P_N+3is (Cb(N+3),Cr(N+3)). The color difference value Î”C_Nof the target pixel P_Nis determined according to the region(s) where the coordinate (Cb(Nâˆ’3),Cr(Nâˆ’3)) of the first pixel P_Nâˆ’3and the coordinate (Cb(N+3),Cr(N+3)) of the second pixel P_N+3are located. In this embodiment, when the coordinate (Cb(Nâˆ’3),Cr(Nâˆ’3)) of the first pixel P_Nâˆ’3and the coordinate (Cb(N+3),Cr(N+3)) of the second pixel P_N+3are located in the same region, e.g. the region R1, the color difference value Î”C_Nof the target pixel P_Nequals 0. When the coordinates (Cb(Nâˆ’3),Cr(Nâˆ’3)) and (Cb(N+3),Cr(N+3)) are located in two different regions, e.g. the coordinate (Cb(Nâˆ’3),Cr(Nâˆ’3)) is located in the region R1 while the coordinate (Cb(N+3),Cr(N+3)) is located in the region R2 and neither of the two coordinates is located in the overlap areas 301 and 302 of the regions R1 and R2, the color difference value Î”C_Nequals the distance between the two coordinates. In other words, the color difference value Î”C_Nequals âˆš{square root over ((Cb(Nâˆ’3)âˆ’Cb(N+3))²+(Cr(Nâˆ’3)âˆ’Cr(N+3))²)}{square root over ((Cb(Nâˆ’3)âˆ’Cb(N+3))²+(Cr(Nâˆ’3)âˆ’Cr(N+3))²)}{square root over ((Cb(Nâˆ’3)âˆ’Cb(N+3))²+(Cr(Nâˆ’3)âˆ’Cr(N+3))²)}{square root over ((Cb(Nâˆ’3)âˆ’Cb(N+3))²+(Cr(Nâˆ’3)âˆ’Cr(N+3))²)}. For simplicity, when the coordinates (Cb(Nâˆ’3),Cr(Nâˆ’3)) and (Cb(N+3),Cr(N+3)) are located in two different regions, the color difference value Î”C_Ncan also be equal to (|Cb(Nâˆ’3)âˆ’Cb(N+3)|+|Cr(Nâˆ’3)âˆ’Cr(N+3)|).

Please refer to FIG. 1. The processing unit 105 first calculates the color difference values Î”C of each of the pixels. Then, the weight generator 107 coupled to the processing unit 105 receives the color difference values Î”C and generates corresponding weighting values. In this embodiment, the weighting value is a monotonic decreasing function of the color difference value Î”C. In other words, the larger the color difference value Î”C_N, the smaller the weighting value of the target pixel P_N. In this embodiment, the weight generator 107 interpolates the weighting values based on two predetermined color difference values and two corresponding weighting values.

Please refer to FIG. 4, which shows the relationship between the color difference value and the weighting value according to an embodiment of the present invention. In this embodiment, the weight generator 107 uses three predetermined color difference values D1ËœD3 and three corresponding weighting values W1ËœW3 as bases for interpolating a weighting value W_Ncorresponding to a color difference value D_N, where D₃>D₂>D₁â‰§0, and W₁>W₂>W₃â‰§0. For example, when the color difference value D_Nlies between the color difference values D₂and D₃, the weight generator 107 can determine the weighting value W_Naccording to the following equations:

$\frac{D_{N} - D_{2}}{W_{N} - W_{2}} = \frac{D_{3} - D_{2}}{W_{3} - W_{2}}$ $W_{N} = W_{2} + (D_{3} - D_{2}) Ã— \frac{D_{N} - D_{2}}{W_{3} - W_{2}}$

Similarly, when the color difference value D_Nlies between the color difference values D₁and D₂, the weight generator 107 can determine the weighting value W_Naccording to the following equation:

$\frac{D_{N} - D_{1}}{W_{N} - W_{1}} = \frac{D_{2} - D_{1}}{W_{2} - W_{1}}$

In another embodiment, the weight generator 107 can utilize a lookup table to determine the weighting value of a target pixel according to the color difference value of the target pixel. The lookup table can be built according to the relationship between the two values as shown in FIG. 4. With the weighting values corresponding to various color difference values recorded in the lookup table, the weight generator 107 can lookup a weighting value of a pixel according to the color difference value of the pixel. The weight generator 107 then generates a weighting signal SW that includes the weighting values of the pixels.

Please refer back to FIG. 1. The sharpening circuit 103 is coupled to the adaptor 101 to receive the weighting signal S_W. The sharpening circuit 103 performs a sharpening operation on the input video signal S_INaccording to the weighting signal S_Wto generate the sharpened video signal S_OUT. Please refer to FIG. 5, which is a block diagram of the sharpening circuit 103 and the adaptor 101 according to another embodiment of the present invention. The sharpening circuit 103 includes a first multiplier 109, a high-pass filter 111, a coring circuit 113, a second multiplier 115, a delay unit 117, and an adder 119. The first multiplier 109 is coupled to the adaptor 101 to receive the weighting signal S_W. The first multiplier 109 multiplies each weighting value in the weighting signal S_Wby a gain S_Gto generate a total weighting signal S_T.

The high-pass filter 111 receives the input video signal S_INand high-pass filters the input video signal S_INto generate a high frequency video signal S_H. The coring circuit 113 is coupled to the high-pass filter 111 to receive the high frequency video signal S_H. The coring circuit 113 eliminates a core portion of the high frequency video signal S_Hto generate a coring video signal S_C. Please refer to FIG. 6, which shows a partial timing diagram of the high frequency video signal according to an embodiment of the present invention. In this embodiment, the coring circuit 113 defines the core portion of the high frequency video signal S_Hto be between thresholds T and âˆ’T. A person of ordinarily skills in the art can modify the definition of the core portion by altering the value of the threshold T. FIG. 7 is a partial timing diagram of the coring video signal according to an embodiment of the present invention. As shown in FIG. 7, the high frequency video signal S_His changed into the coring video signal S_Cafter the coring circuit 113 eliminates the core portion of the high frequency video signal S_H.

The second multiplier 115 is coupled to the first multiplier 109 and the coring circuit 113. The second multiplier 115 multiplies the coring video signal S_Cby the total weighting signal S_Tto generate a sharpened signal S_A. The delay unit 117 receives the input video signal S_INand delays the input video signal S_INto generate a delayed video signal S_D. The adder 119 is coupled to the second multiplier 115 and the delay unit 117. The adder 119 adds up the sharpened signal S_Aand the delayed video signal S_Dto generate a sharpened video signal S_OUT.

A person of ordinarily skills in the art is able to recognize that the sharpened signal S_Acan enhance the contrast, e.g. difference in brightness, between the target pixel P_Nand its adjacent pixel(s), which is not construed as a limitation to this application. In this embodiment, by multiplying the coring video signal S_Cby the total weighting signal S_T, the second multiplier 115 enhances the difference between the target pixel P_Nand its adjacent pixel(s) in not only brightness but also darkness. In another embodiment, the second multiplier 115 can multiply only the positive amplitudes of the coring video signal S_Cby the weighting signal S_Tto decrease the difference between the target pixel P_Nand its adjacent pixel(s) in only brightness. In yet another embodiment, the second multiplier 115 can multiply only the negative amplitudes of the coring video signal S_Cby the weighting signal S_Tto decrease the difference between the target pixel P_Nand its adjacent pixel(s) in only darkness.

Please refer to FIG. 8, which shows another embodiment of the sharpening circuit 103 of FIG. 1. As shown in FIG. 8, the sharpening circuit 103a includes a first multiplier 109, a low-pass filter 121, a coring circuit 113, a second multiplier 115, a delay unit 117, and a subtractor 123. Since the sharpening circuit 103a in FIG. 8 is similar to the sharpening circuit 103 in FIG. 5, the following description will focus on the difference between them. Please refer to FIG. 8, the low-pass filter 121 receives the input video signal S_INand low-pass filters the input video signal S_INto generate a low frequency video signal S_L. The coring circuit 113 is coupled to the low-pass filter 121 to receive the low frequency video signal S_L. The coring circuit 113 eliminates a core portion of the low frequency video signal S_Lto generate a coring video signal S_C. The subtractor 123 is coupled to the second multiplier 115 and the delay unit 117. The subtractor 123 subtracts the sharpened signal S_Afrom the delayed video signal S_Dto generate a sharpened video signal S_OUT.

Please refer to FIG. 9, which shows another embodiment of the sharpening circuit 103 of FIG. 1. As shown in FIG. 9, the sharpening circuit 103b includes a high-pass filter 111, a coring circuit 113, a second multiplier 115, a delay unit 117, and an adder 119. Since the sharpening circuit 103b in FIG. 9 is similar to the sharpening circuit 103 in FIG. 5, the following description will focus on the difference between them. Unlike the sharpening circuit 103, the sharpening circuit 103b does not include a first multiplier 109. The second multiplier 115 is coupled to the weight generator 107 and the coring circuit 113 directly. The second multiplier 115 multiplies the coring video signal S_Cby the weighting signal S_Wto generate the sharpened signal S_A.

Please refer to FIG. 10, which shows yet another embodiment of the sharpening circuit 103 of FIG. 1. The sharpening circuit 103c includes a low-pass filter 121, a coring circuit 113, a second multiplier 115, a delay unit 117, and a subtractor 123. Since the sharpening circuit 103c in FIG. 10 is similar to the sharpening circuit 103a in FIG. 8, the following description will focus on the difference between them. Unlike the sharpening circuit 103a, the sharpening circuit 103c does not include a first multiplier 109. The second multiplier 115 is coupled to the weight generator 107 and the coring circuit 113 directly. The second multiplier 115 multiplies the coring video signal S_Cby the weighting signal S_Wto generate the sharpened signal S_A.

In addition to the aforementioned embodiments, an image processing method is described in the following paragraphs. Please refer to FIG. 11, which is a flow chart of the image processing method according to an embodiment of the present invention. The image processing method in this embodiment includes steps S1101, S1103, and S1105. In step S1101, an input video signal that includes the information of a plurality of pixels is received, and the input video signal is processed to determine the color difference values of the pixels. The color difference value of each target pixel is determined according to the chromatic information of at least two pixels adjacent to the target pixel.

In this embodiment, the chromatic information of each pixel includes a first chrominance Cb and a second chrominance Cr. The color difference value of a target pixel is determined according to the first and the second chrominances of a first pixel and the first and the second chrominances of a second pixel. The first and the second pixels are adjacent to the target pixel, and the target pixel is located on the middle point of the first and the second pixels. Hereinafter, Cb(1) and Cr(1) are used to represent the first and the second chrominances of the first pixel, respectively; Cb(2) and Cr(2) are used to represent the first and the second chrominances of the second pixel, respectively. The color difference value of the target pixel equals (|Cb(1)âˆ’Cb(2)|+|Cr(1)âˆ’Cr(2)|)/2.

According to an embodiment of the present invention, the color difference value of the target pixel is calculated as follows. In a chromatic coordinate system, the first chrominance Cb and the second chrominance Cr are used as reference axes, and there are four regions: {(Cb, Cr)| Cbâ‰¦Cb2, Crâ‰¦Cr2}, {(Cb, Cr)| Cbâ‰§Cb1, Crâ‰¦Cr2}, {(Cb, Cr)| Cbâ‰¦Cb2, Crâ‰§Cr1}, and {(Cb, Cr)|Cbâ‰§Cb1, Crâ‰§Cr1}, where Cb2>Cb1>0 and Cr2>Cr1>0. In the embodiment, the color difference value is determined according to region(s) of the chromatic coordinate system where the coordinates (Cb(1),Cr(1)) and (Cb(2),Cr(2)) are located. For example, when the coordinates (Cb(1),Cr(1)) and (Cb(2),Cr(2)) lie in the same region of the four regions, the color difference value equals 0. When the coordinates (Cb(1),Cr(1)) and (Cb(2),Cr(2)) lie in two different regions of the four regions, the color difference value equals âˆš{square root over ((Cb(1)âˆ’Cb(2))²+(Cr(1)âˆ’Cr(2))²)}{square root over ((Cb(1)âˆ’Cb(2))²+(Cr(1)âˆ’Cr(2))²)}{square root over ((Cb(1)âˆ’Cb(2))²+(Cr(1)âˆ’Cr(2))²)}{square root over ((Cb(1)âˆ’Cb(2))²+(Cr(1)âˆ’Cr(2))²)}. In another embodiment, when the coordinates (Cb(1),Cr(1)) and (Cb(2),Cr(2)) lie in two different regions of the four regions, the color difference value equals (|Cb(1)âˆ’Cb(2)|+|Cr(1)âˆ’Cr(2)|).

In step S1103, the weighting signal is generated according to the color difference values of the pixels. The weighting signal includes the weighting values of the pixels, and the weighting value of each pixel is generated according to the color difference value of the pixel. In this embodiment, the weighting value is a monotonic decreasing function of the color difference value. There are several sub-steps of step S1103 which can be adopted to calculate the weighting values. For example, with at least two predetermined color difference values and corresponding weighting values, the weighting values of the pixels can be interpolated in step S1103 according to the predetermined values. A lookup table can also be utilized in step S1103 to lookup the weighting values for the pixels.

In step S1105, a sharpening operation is performed on the input video signal according to the weighting signal to generate the sharpened video signal. Please refer to FIG. 12, which is a flow chart showing how the sharpened video signal is generated according to an embodiment of the present invention. In step S1201, the weighting value of each of the pixels is multiplied by a gain. In step S1203, a sharpening operation is performed on the input video signal according to the weighting signal and the gain to generate the sharpened video signal.

Please refer to FIG. 13, which shows another way to generate the sharpened video signal. In step S1301, the input video signal is high-pass filtered to generate a high frequency video signal. In step S1303, the core portion of the high frequency video signal is eliminated to generate a coring video signal. In step S1305, the coring video signal is multiplied by the weighting signal to generate a sharpened signal. In step S1307, the input video signal is delayed to generate a delayed video signal. In step S1309, the sharpened signal and the delayed video signal are added up to generate the sharpened video signal.

Please refer to FIG. 14, which shows yet another way to generate the sharpened video signal. In step S1401, the input video signal is low-pass filtered to generate a low frequency video signal. In step S1403, the core portion of the low frequency video signal is eliminated to generate a coring video signal. In step S1405, the coring video signal is multiplied by the weighting signal to generate a sharpened signal. In step S1407, the input video signal is delayed to generate a delayed video signal. In step S1409, the sharpened signal is subtracted from the delayed video signal to generate the sharpened video signal.

As mentioned above, the processing unit 105 receives the input video signal S_INand then determines the color difference values Î”C of the pixels in the input video signal S_IN. In one embodiment, the processing unit 105 determines the color difference value Î”C of a target pixel according to the chromatic information of at least two pixels adjacent to the target pixel. The chromatic information of each pixel includes a first chrominance Cb and a second chrominance Cr. Generally, the pixels lying in the border area of different colors have larger color difference values. According to the color difference values Î”C generated by the processing unit 105, the weight generator 107 generates a weighting signal S_W. For example, according to the color difference value Î”C of each pixel, the weight generator 107 performs an interpolation or a table-lookup operation to determine the weighting value of the pixel. To shun undesired effects, such as halos, on color borders, a larger color difference value Î”C should correspond to a smaller weighting value. The weighting signal S_Wincludes the weighting values of the pixels. The sharpening circuit 103 adjusts the degree of the sharpening operation performed on the input video signal S_INaccording to the weighting signal S_W. The resulted sharpened video signal S_OUTthen has a better visual quality than that of the input video signal S_IN.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or the spirit of the invention. In view of the foregoing, it is intended that the present invention covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.